Aluminum electrolytic capacitors are widely used because they allow a high capacity to be achieved at a low cost. Aluminum foil is generally used as an electrode material for an aluminum electrolytic capacitor.
The surface area of the electrode material for an aluminum electrolytic capacitor can usually be increased by performing an etching treatment to form etching pits. The etched surface of the electrode material is then anodized to obtain an oxide film that functions as a dielectric substance. By etching the aluminum foil and applying one of various voltages to the surface thereof to match the voltage that is to be used, an anodic oxide film can be formed, thus enabling various aluminum anodes (foils) to be produced for electrolytic capacitors that are suited to specific applications.
In the etching process, pores called etching pits are formed in the aluminum foil, and the etching pits are processed into various shapes depending on the anodization voltage to be applied.
More specifically, a thick oxide film must be formed for use in medium- to high-voltage capacitors. Therefore, in order to prevent the etching pits from being buried by such a thick oxide film, the etching pits for an aluminum foil that is to be used in a medium- to high-voltage anode are made to a tunnel type by conducting direct-current etching, and then processed to have an appropriate size for the voltage that is to be used. In contrast, small etching pits are necessary for use in low-voltage capacitors. Therefore, sponge-like etching pits are generally formed by alternating-current etching. In a cathode foil, the surface area is similarly increased by etching.
However, the etching treatments for both anodes and cathodes require the use of an aqueous hydrochloric acid solution that contains sulfuric acid, phosphoric acid, nitric acid, etc., in hydrochloric acid. Hydrochloric acid has a strong environmental impact, and its disposal also impacts the production process or production cost. Therefore, the development of a novel method for increasing the surface area of an aluminum foil that does not require etching is in demand.
In order to meet this demand, the use of an aluminum electrolytic capacitor that is characterized by adhering a fine aluminum powder to the surface of an aluminum foil has been proposed (Patent Document 1). Another example of a known electrolytic capacitor is one that uses an electrode foil that comprises a flat aluminum foil having a thickness of not less than 15 μm but less than 35 μm, wherein an aggregate of self-similar aluminum fine particles having a length of 2 to 0.01 μm and/or an aggregate of aluminum fine particles having an aluminum oxide layer formed on the surface thereof are adhered to one or both surfaces of the flat aluminum foil (Patent Document 2).
However, the methods disclosed in the aforementioned documents, wherein aluminum powder is adhered to the aluminum foil by plating and/or vacuum evaporation, are insufficient, at least for obtaining thick etching pits for medium- to high-voltage capacitors.
Patent Document 1: Japanese Unexamined Patent Publication No. 2-267916
Patent Document 2: Japanese Unexamined Patent Publication No. 2006-108159